Technical & economic evaluation of a mineral carbonation process using southern Québec mining wastes for CO2 sequestration of raw flue gas with by-product recovery
Introduction
The Intergovernmental Panel for Climate Change (IPCC) has confirmed in its latest report for policymakers that the increase of greenhouse gases (GHG) emissions is linked with observed changes in climate (IPCC, 2013). Worldwide efforts to reduce emissions should undeniably lead to restrictive legislation to achieve emissions targets. In 2008 the province of Québec joined the Western Climate Initiative (WCI), which also includes the provinces of Ontario, Manitoba, British Columbia (Canada) and the state of California (USA), to implement a cap and trade system to reduce GHG emissions (Gouvernement du Québec, 2013b). Since the beginning of 2013, over 80 facilities from the industrial and power generation sector have been required to comply with GHG emissions regulations. Significant emitters must find solutions to reduce their emissions to avoid binding carbon taxes.
The integration of a CO2 capture and storage process to treat post-combustion gaseous emissions represents a major challenge in terms of GHG reduction. Two different types of storage processes could be applied. 1) Geological storage, where the CO2 is injected into a geological formation and 2) mineral carbonation, where CO2 is chemically reacted with alkaline earth metals above ground in order to form stable and inert carbonates (IPCC, 2005). In this case, the gaseous stream (pure CO2 or a flue gas) is reacted with solid material containing divalent cations such as calcium and magnesium to bind CO2 to form carbonates. Such reactions can be realized in both aqueous and dry conditions. Nevertheless, while mineral carbonation offers a safe sequestration solution, the reaction kinetics can be unreasonably slow (IPCC, 2005). The resulting high operating costs stand as a major disadvantage for further industrial application.
One of the first works published on aqueous mineral carbonation was conducted by Lackner et al. (1995), who suggested this approach as an alternative to geological sequestration. Later work by Huijgen (2003) focused on reacting a supercritical pure CO2 stream under aqueous conditions to produce stable carbonates. Reactions were conducted with pure minerals such as olivine serpentine and wollastonite at high pressure and temperature. The costs associated with feedstock mining and extraction, preconditioning (grinding and milling), thermal activation and extreme operating conditions resulted in uncompetitive economics (O’Connor et al., 2005, Sipilä et al., 2008). Nevertheless, research continued to search for alternatives to increase efficiencies and to reduce operating costs. Among these options, the use of alkaline industrial wastes such as cement kiln dust, coal fly ashes, steel slag or mining tailings was shown to offer opportunities as a more readily reactive feedstock, which in some cases was already finely ground (Bobicki et al., 2012, Bonenfant et al., 2008, Katsuyama et al., 2005, Ukwattage et al., 2013). With such an approach, the impact of solid material preconditioning could be avoided or reduced.
Initial work in the field focused on the reaction of dry carbon dioxide with alkaline feedstock and specifically calcium-containing feedstocks (Zevenhoven et al., 2002). However, due to the slow reaction kinetics of the gas phase reaction, workers have more recently focused on aqueous phase carbonation. In particular, heat treated serpentinite has proved to be an interesting media for aqueous mineral carbonation at low CO2 partial pressures (Werner et al., 2013).
Mineral carbonation costs, based on treatment with a pure CO2 stream, have been estimated to be between 50 and 100 US$/t CO2 (IPCC, 2005, Lackner, 2002, O’Connor et al., 2005, Penner et al., 2004). However, such cost evaluations generally do not take into account the price of the capture step. Post-combustion industrial gases contain relatively low concentrations of CO2 (0–25 vol.% CO2) at low pressure and the costs associated with concentration of this carbon dioxide prior to the carbonation step have an important impact on the global processing economics. Despite technology improvements, costs for post-combustion capture from flue gases using amine solvents can range from $US 52 to 61 per tonne of CO2 avoided (NETL, 2012). This cost needs to be added to the overall cost of sequestration.
The Québec Province is unique due to the large quantities of mining tailings generated by chrysotile asbestos exploitation in the southern region. An estimated 2 Gt of magnesium silicate residues is available in this province and could sequester 625 Mt of CO2 (Huot et al., 2003). Natural carbonation of such tailings is known and a number of studies have been conducted to measure and increase the direct carbonation of the tailings in situ (Wilson et al., 2009, Wilson et al., 2006). Recent studies have shown that such natural carbonation rates may have been underestimated and that this process could offset the GHG generated by the mining operations (Harrison et al., 2012). In addition, using such feedstock types can also reduce health issues associated with the degradation of asbestos fibers (Gadikota et al., 2014). On the other hand, the recent economic study presented by Hitch and Dipple (2012) showed that the implementation of a sequestration process using the carbonation of such residues to offset emissions was financially feasible with the establishment of a cap and trade system. Moreover, recent studies by Kirchofer et al. (2012) and Giannoulakis et al. (2014) have presented life cycle assessments for mineral carbonation technologies for the US and European context respectively. Both studies considered the use of serpentine minerals in high temperature and pressure processes as described by Huijgen et al. (2007). While exploitation of chrysotile fibers in Quebec is now abandoned, the residues still stand as a potential sequestration media due to the advantageous geographic situation of the tailings compared to large emission sources.
This article focuses on the economic evaluation of a process using heat-treated mining residues for reducing CO2 emissions from industrial flue gases. The GHG balance for the overall process is also determined. This paper does not pretend to present an exhaustive analysis. Nevertheless, it identifies potential sensitivities regarding the implementation of such a process at an industrial site within Quebec and provides guidance for the development of mineral carbonation as a viable sequestration technique elsewhere in the world.
Section snippets
Process diagram
This study focuses on the use of mining residues from the Thetford-Mines and Asbestos areas in the Province of Québec. As previously described, the history of the region has been mainly focused on the exploitation of chrysotile asbestos. On a smaller scale, chromite was also extracted in the area. Thus, the composition of the mining residues varies depending on the location. The parameters used for the simulations were taken from experiments conducted with residues extracted from the old
Mass balance
The process mass balances were calculated based on the experimental results obtained during the laboratory batch tests presented in Pasquier et al. (2014). Fig. 2 shows the mass balance for both CO2 and MgO calculated on the basis of 1 t of rock treated in the whole process, including the grinding, magnetic separation and the first heat activation steps. Thus, because of the mass loss induced by removing the magnetic fraction (10 wt.%) and the heat activation, where there is a further 10 wt.%
Discussion
The theoretical energy and economic analysis of the process gives different information relative to the possible implementation of the technology. First the energy demand of the process is an important point to consider. The overall carbonation process requires 3 kJ/t of rocks treated. Another important point highlighted by the results is the importance of the energy source. The use of electricity generated from coal or NG has a negative impact on the process sequestration efficiency. Even if
Conclusion
This study focused on the evaluation of the energy requirements and the associated costs of a mineral carbonation technology. The use of available utlramafic mining residues for the treatment of 18 vol.%CO2 flue gas was modeled based on laboratory scale results. Following the process flowsheet, the equipment requirements and their energy demand was evaluated. The comparison of the presented results showed a significant energy reduction for direct flue gas treatment compared with a pure CO2
Acknowledgement
The present research was funded by le Fond de Recherche Québécois en Nature et Technologies and Carbon Management Canada.
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